CSEP Position Stand - Canadian Society for Exercise Physiology

547
POSITION STAND / PRISE DE POSITION
Canadian Society for Exercise Physiology position
paper: resistance training in children and
adolescents
David G. Behm, Avery D. Faigenbaum, Baraket Falk, and Panagiota Klentrou
Abstract: Many position stands and review papers have refuted the myths associated with resistance training (RT) in children and adolescents. With proper training methods, RT for children and adolescents can be relatively safe and improve
overall health. The objective of this position paper and review is to highlight research and provide recommendations in aspects of RT that have not been extensively reported in the pediatric literature. In addition to the well-documented increases
in muscular strength and endurance, RT has been used to improve function in pediatric patients with cystic fibrosis and
cerebral palsy, as well as pediatric burn victims. Increases in children’s muscular strength have been attributed primarily
to neurological adaptations due to the disproportionately higher increase in muscle strength than in muscle size. Although
most studies using anthropometric measures have not shown significant muscle hypertrophy in children, more sensitive
measures such as magnetic resonance imaging and ultrasound have suggested hypertrophy may occur. There is no minimum age for RT for children. However, the training and instruction must be appropriate for children and adolescents, involving a proper warm-up, cool-down, and appropriate choice of exercises. It is recommended that low- to moderateintensity resistance exercise should be done 2–3 times/week on non-consecutive days, with 1–2 sets initially, progressing
to 4 sets of 8–15 repetitions for 8–12 exercises. These exercises can include more advanced movements such as Olympicstyle lifting, plyometrics, and balance training, which can enhance strength, power, co-ordination, and balance. However,
specific guidelines for these more advanced techniques need to be established for youth. In conclusion, an RT program
that is within a child’s or adolescent’s capacity and involves gradual progression under qualified instruction and supervision with appropriately sized equipment can involve more advanced or intense RT exercises, which can lead to functional
(i.e., muscular strength, endurance, power, balance, and co-ordination) and health benefits.
Key words: youth, pediatric, exercise, health, strength.
Résumé : Selon de nombreux énoncés de principe et des articles de synthèse, il n’y a pas lieu d’interdire l’entraı̂nement à
la force (RT) chez les enfants et les adolescents. Si les méthodes sont ajustées à ces groupes de sujets, l’entraı̂nement à la
force s’avère sécuritaire et améliore la santé globale de l’individu. Le but de cet article-synthèse est d’arriver à un énoncé
de principe après avoir analysé les textes de la littérature scientifique et de formuler des recommandations relatives à l’entraı̂nement à la force n’ayant pas fait l’objet d’études poussées dans la recherche pédiatrique. En plus de susciter l’amélioration de la force et de l’endurance musculaires, comme le rapportent de nombreuses études, l’entraı̂nement à la force
améliore les fonctions chez les enfants souffrant de fibrose kystique, de paralysie cérébrale et de brûlures. D’après les études, l’augmentation de la force observée chez les enfants serait surtout due aux adaptations du système nerveux plus qu’à
l’hypertrophie, car les gains observés sont démesurément importants. Bien que dans la plupart des études portant sur les
caractéristiques anthropométriques, on n’a pas observé un degré significatif d’hypertrophie, les études faisant appel à la résonance magnétique et à l’ultrasonographie indiquent que l’hypertrophie est possible. Il n’y a pas d’âge minimal pour s’entraı̂ner à la force quand on est jeune. Ceci étant dit, il faut ajuster le programme d’entraı̂nement et les directives aux
enfants et aux adolescents concernant l’échauffement, le retour au calme et le choix d’exercices. On recommande un entraı̂nement au moyen de charges légères et modérées à raison de 2 à 3 fois par semaine en sautant des jours et en commençant par 1 à 2 séries d’exercices pour en arriver à 4 séries constituées de 8 à 15 répétitions d’un ensemble de 8 à 12
exercices. Parmi ces exercices, on peut inclure des mouvements avancés qu’on observe en haltérophilie aux Jeux olympiques, des exercices pliométriques et des exercices d’équilibre, tous contribuant à l’amélioration de la force, de la puisReceived 30 October 2007. Accepted 24 January 2008. Published on the NRC Research Press Web site at apnm.nrc.ca on 10 April 2008.
D.G. Behm.1 School of Human Kinetics and Recreation, Memorial University of Newfoundland, St. John’s, NL A1C 5S7, Canada.
A.D. Faigenbaum. Department of Health and Exercise Science, The College of New Jersey, P.O. Box 7718, Ewing, NJ 08628-0718,
USA.
B. Falk and P. Klentrou. Department of Physical Education & Kinesiology, Brock University, St. Catharines, ON L2S 3A1, Canada.
1Corresponding
author (e-mail: [email protected]).
Appl. Physiol. Nutr. Metab. 33: 547–561 (2008)
doi:10.1139/H08-020
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Appl. Physiol. Nutr. Metab. Vol. 33, 2008
sance, de la coordination et de l’équilibre. En ce qui concerne ces derniers mouvements, il est important d’élaborer des directives spécifiques à ce groupe de sujets. En conclusion, les enfants et les adolescents peuvent s’entraı̂ner à la force au
moyen d’exercices standards et de niveau avancé pourvu que la progression de l’entraı̂nement soit graduelle et bien supervisée par du personnel qualifié et que l’équipement soit adapté aussi à ce groupe de sujets. Le bilan est positif aux plans
fonctionnels (force musculaire, endurance, puissance, équilibre et coordination) et de la santé.
Mots-clés : jeunesse, pédiatrie, exercice physique, santé, force musculaire.
[Traduit par la Rédaction]
______________________________________________________________________________________
Definition of terms
For the purpose of this paper, the term ‘‘children’’ refers
to boys and girls who have not yet developed secondary sex
characteristics (approximately up to age 11 in girls and 13 in
boys; Tanner stages 1 and 2 of sexual maturation). This period of development is often referred to as pre-adolescence.
The term ‘‘adolescence’’ refers to the period of time between childhood and adulthood and includes girls aged 12–
18 years and boys aged 14–18 years (Tanner stages 3 and 4
of sexual maturation). The term ‘‘youth’’ is broadly defined
in this paper to include the years of childhood and adolescence. The term ‘‘resistance training’’ refers to a specialized
method of conditioning that involves the progressive use of
a wide range of resistive loads, including body mass, and a
variety of training modalities designed to enhance health,
fitness, and sports performance. Although the terms ‘‘resistance training’’, ‘‘strength training’’, and ‘‘weight training’’ are sometimes used synonymously, resistance training
encompasses a broader range of training modalities and a
wider variety of training goals. The term ‘‘weightlifting’’ refers to a competitive sport that involves the snatch and clean
and jerk lifts.
Introduction
The conclusions regarding the beneficial effects of resistance training (RT) for pre-adolescent children and adolescents has been consistently positive in the scientific
literature. The concerns and myths that were pervasive
throughout the general population have been persistently refuted in the scientific literature. Some of these myths purported that RT for children would result in stunted growth,
epiphyseal plate damage, lack of strength increases due to a
lack of testosterone, and a variety of safety issues (Blimkie
1993). There has been a universal acceptance in various association position papers (American Academy of Pediatrics
2001; American College of Sports Medicine 2006; British
Association of Sport and Exercise Science 2004; Faigenbaum
et al. 1996b; Golan et al. 1998; Smith et al. 1993) and review articles (Blimkie 1993, 1992; Faigenbaum 2000; Falk
and Eliakim 2003; Falk and Tenenbaum 1996; Hass et al.
2001; Malina 2006; McNeely and Armstrong 2002; Payne
et al. 1997; Sale 1989; Webb 1990) that RT for children
will improve muscular strength and muscular endurance if
performed under the supervision of a qualified instructor,
using proper technique, gradual training progressions, and
proper warm-up and cool-down periods. These strength
gains are relatively comparable with adolescent or adult
strength gains, but do not typically provide substantial
gains in muscle size (Blimkie 1993, 1992). Falk and
Tenenbaum (1996) conducted a meta-analysis and reported
RT-induced strength increases of 13%–30% in preadolescent children following RT programs of 8–20 weeks.
Rather than contributing to injuries as was previously
thought, RT has been reported to be safe (when supervised
and with proper technique) for children and to potentially
decrease the incidence and severity of sport injuries
(Faigenbaum et al. 1996b; Falk and Eliakim 2003; Hamill
1994; McNeely and Armstrong 2002; Smith et al. 1993;
Webb 1990). Furthermore, RT has been reported to increase bone mineral density (Nichols et al. 2001) while
not adversely affecting maturational growth (Sadres et al.
2001), cardiorespiratory fitness, or resting blood pressure
(Blimkie 1993), and either has no effect on or improves
body composition (Faigenbaum et al. 1993; Hass et al.
2001; Lillegard et al. 1997; Sadres et al. 2001; Siegal et
al. 1989; Sothern et al. 2000). In addition, RT can have a
positive effect on other health- and fitness-related measures
(Faigenbaum 2000) including the blood lipid profile (Hass
et al. 2001). Psychosocial skills and measures of well
being can be enhanced with RT (Faigenbaum et al. 1996b;
Falk and Eliakim 2003; Hass et al. 2001), as can motor
control skills or performance (Faigenbaum 2000; Falk and
Eliakim 2003; Hass et al. 2001) and co-ordination (Blimkie
1993). Although there is some diversity of opinion on
whether sports performance is directly improved with RT,
it appears that regular participation in a sport-specific
resistance-training program can result in some degree of
improvement in athletic performance in young athletes
(Faigenbaum et al. 1996b; Falk and Eliakim 2003;
McNeely and Armstrong 2002; Webb 1990). The current
literature generally agrees that low- to moderate-intensity
resistance should be conducted (Golan et al. 1998; Hass et
al. 2001) 2–3 times/week on non-consecutive days, with 1–
4 sets of 6–20 repetitions for 6–12 exercises and generally
through a full range of motion (Faigenbaum et al. 1996b;
Golan et al. 1998; Malina 2006; McNeely and Armstrong
2002; Webb 1990). Thus, if it is now well accepted among
sports and medical associations that RT is effective and
beneficial for children and adolescents, is there any necessity for another position paper or review on this matter?
Although the benefits and prescriptions for standard RT
programs are well established, there are a number of relatively new or more advanced RT concepts that have not
been comprehensively addressed in the pediatric literature.
More advanced training concepts such as plyometrics, instability RT, periodization, Olympic-style weightlifting, testing
methods, and others have been well documented in the adult
literature but have received much less exposure or research
in the pediatric literature and may be somewhat controver#
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Behm et al.
sial. It is important to highlight new knowledge in these
areas or alert the professionals to the lack of information
and the possibility of future research directions in the area
of pediatric RT. Thus, it is the objective of this position paper to highlight the major findings related to new trends in
pediatric RT and the benefits and the mechanisms underlying the training adaptations in children, to provide training
recommendations, and to illustrate areas that need more research.
Health benefits
In the past, RT was not recommended for children as it
was believed to be ineffective in terms of strength improvements, while at the same time thought to lead to injuries and
long-term health consequences such as damage of growth
plates and premature closure of epiphyses. However, recent
studies are finding positive results with such practice and
have proven RT to actually be beneficial to this population
(Steinberger 2003). There is actually an increasing amount
of evidence suggesting that RT has the potential to increase
bone mineral density, develop greater muscle strength and
endurance, and maintain lean body mass, as well as provide
a rehabilitation vehicle for various other conditions that impair growth such as cystic fibrosis and osteopenia in both
pre- and post-adolescent youth. RT can also lead to improvements in motor skills and performance while helping
resist injury and building up a positive attitude by increasing
confidence levels and self-esteem (Faigenbaum 2007; Hass
et al. 2001; Suman et al. 2001). Accordingly, strong emphasis relies upon ensuring proper technique and considering
confounding variables, such as the type and length of the
training program. For optimal outcome, the RT program
should be designed specifically in conjunction with the age,
gender, health status, and physical fitness of the child involved. Table 1 presents a summary of studies on the
health-related effects of RT in children and adolescents.
Muscular strength and endurance of children and adolescents have been shown to significantly improve beyond normal growth and maturation when practicing a specifically
designed RT program (Benson et al. 2007; Faigenbaum et
al. 1999; Falk and Mor 1996; Ramsay et al. 1990). To control for growth and maturation effects, the majority of RT
studies included an age-matched control group and have
shown that over a period of 6–20 weeks, muscle strength
and performance increased to a greater extent in children
who participated in RT compared with those who did not.
More specifically, it has been reported that moderate loads
(e.g. 50%–60% of one repetition maximum (1RM)) and
higher repetitions (e.g. 15–20 repetitions) may be most beneficial for enhancing muscular strength and endurance in
youth during the initial adaptation period (Benson et al.
2007; Christou et al. 2006; Faigenbaum et al. 1999, 2005b;
Lillegard et al. 1997; Pfeiffer and Francis 1986). Overall, in
a recent summary by Malina (2006), the 22 reviewed studies
agreed that RT twice or three times per week resulted in significant improvements in muscle strength during childhood
and adolescence, and that minimal injuries were reported.
Significant gains have been reported in isometric and isokinetic strength, muscular endurance, and flexibility with
RT protocols of different frequencies and duration, and
549
across maturity levels (Blimkie et al. 1989; Faigenbaum et
al. 1993, 1999, 2001, 2005b; Rians et al. 1987; Ramsay et
al. 1990; Sailors and Berg 1987; Weltman et al. 1986).
These training-induced improvements were in some cases
more evident in older boys and greater in lower- than in
upper-body strength, and with 2 days/week compared to
1 day/week (Pikosky et al. 2002; Vrijens 1978; Faigenbaum
et al. 2002). A 12-week school-based RT program also resulted in significant improvements of strength, endurance,
and flexibility in pre-pubertal boys and girls as compared
with their control counterparts (Siegal et al. 1989). In addition, an injury-free 12-week combined program of resistance
and martial arts exercises showed improvements in physical
performance tasks reflecting muscle strength, endurance,
power, and coordination (Falk and Mor 1996). Strength
training can also augment the muscle enlargement that normally occurs with pubertal growth in males and females
(Kraemer et al. 1989; Webb 1990), but the magnitude of
changes in children’s cross-sectional muscle area is smaller
than that found in adults (Fukunaga et al. 1992; Mersch and
Stoboy 1989). It should also be noted that gains in muscle
strength and power begin to regress towards untrained values if RT is discontinued (Faigenbaum et al. 1996a;
Tsolakis et al. 2004).
Bone health is another area of study when considering
health benefits of RT (Table 1). Bass et al. (1998) have reported that pre-pubertal female gymnasts, whose training
mainly involves high-impact and resistance training, had significantly higher bone mineral density (BMD) than agematched controls. Lumbar spine bone mass, volume, and
volumetric BMD were also higher in the gymnasts than
those found in the control group. They also showed that
endocortical diameter was lower in the control group, suggesting an increased cortical thickness in the gymnasts. The
gymnasts, however, were growing at a slower rate than the
controls when comparing sitting height, femur height and
length, and tibia length. This does not seem to be related to
the training but is rather a result of selection because of an
advantage of shorter athletes in the sport (Daly et al. 2000;
Erlandson et al. 2008; Gurd and Klentrou 2003). In a more
recent study, Ward et al. (2005) also compared the bone size
of the peripheral and axial skeleton among pre-pubertal
gymnasts, swimmers, and controls. After adjustment for age
and gender, they found that male pre-pubertal gymnasts had
significantly thicker cortical bone at the tibia and radius than
the controls (Ward et al. 2005). Adolescent male weightlifters have also been found to have significantly greater
BMD or bone mineral content (BMC) than age-matched
controls (Conroy et al. 1993; Virvidakis et al. 1990). Conroy
et al. (1993) have shown a significant relationship between
BMD and muscle strength in this group of junior weightlifters, with strength accounting for 30%–65% of variance,
whereas in the Virvidakis et al. (1990) study, BMC was
highly correlated with weight record. Furthermore, Nichols
et al. (2001) compared a group of 13–17 years old females
assigned to an RT intervention group 3 times/week for
15 months with a control group of age-matched females.
They reported no significant changes in the lumbar BMD
and BMC in their RT group as compared with the control
group. The only difference between the groups was an increased leg strength and femoral neck BMD in the RT
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Table 1. Summary of the effects of resistance training (RT) in children and adolescents.
Effect
Muscle strength
Children
+++
Adolescents
+++
Sample references
Blimkie et al. 1989, 1996; Christou et al. 2006;
Faigenbaum et al. 1993, 1996a, 2001, 2002,
2005b; Fukunaga et al. 1992; Lillegard et al. 1997;
Nichols et al. 2001; Ozmun et al. 1994; Pfeiffer
and Francis 1986; Pikosky et al. 2002; Ramsay et
al. 1990; Sadres et al. 2001; Sailors and Berg
1987; Siegal et al. 1989; Tsolakis et al. 2004;
Weltman et al. 1986
Christou et al. 2006; Faigenbaum et al. 1993, 1996a,
2002, 2005b; Lillegard et al. 1997; Weltman et al.
1986
Faigenbaum et al. 1999, 2001, 2005b; Ramsay et al.
1990; Sailors and Berg 1987
Blimkie et al. 1996; Nichols et al. 2001
Muscle power
?
+
Muscular endurance
++
+
Bone strength,
BMD, BMC
?
?
Flexibility
+
?
Christou et al. 2006; Faigenbaum et al. 2002, 2005b;
Siegal et al. 1989; Weltman et al. 1986
Agility and physical
performance
?
?
Christou et al. 2006; Falk and Mor 1996
Body composition
—
?
Faigenbaum et al. 1993; Lillegard et al. 1997;
Sadres et al. 2001; Siegal et al. 1989; Sothern et
al. 2000; Siegal et al. 1989
Notes
Smaller absolute strength
gains in children compared
with adults, but comparable
relative gains
Small if any changes in
children; limited data in
adolescents
Limited data in adolescents
Limited number of studies
using RT alone to examine
effect on bone
Small if any changes in
children; limited data in
adolescents
Changes only shown when
RT was combined with
specific sports training
Some data suggesting reduced
adiposity in overweight
children; no data in
adolescents
Note: +++, clear effect in numerous studies; ++, some effect in limited number of studies; +, small effect in limited number of studies; ?, unclear effect; —, no effect. BMD, bone mineral density; BMC, bone mineral content.
group. Based on the skeletal benefits described (Table 1),
RT beginning at a young age is also associated with a decreased risk of osteoporotic fractures later in life (Heinonen
et al. 2000). Childhood through late adolescence is a crucial
period in bone formation, with about 50% of the peak bone
mass being acquired during this period (Bonjour et al. 1991;
Matkovic et al. 1994). Peak bone mass is defined as the
amount of bony tissue present at the end of skeletal maturation. Because a low peak bone mass is a significant risk factor for osteoporosis and associated fractures, the attainment
of an ample peak bone mass during childhood and adolescence is an effective method to reduce the risk for the later
development of osteoporosis (Hansen et al. 1991).
As the prevalence of childhood obesity continues to increase, the positive impact of RT on body composition in
obese youths should be considered. A number of studies
have been reported that regular participation in RT programs
resulted in an improvement of body composition in obese
children and adolescents (Sothern et al. 2000; Treuth et al.
1998; Watts et al. 2004). RT has also been used as a rehabilitation strategy in children with other chronic conditions.
Selvadurai et al. (2002) studied 3 groups of children suffering from cystic fibrosis with pulmonary exacerbation: a
group who participated in an aerobic-training program, an
RT group, and a control group. They found that both the
aerobic and RT groups had positive results compared with
the control group. More specifically, the RT group had improved lung function, leg muscle strength, and fat-free mass.
Research studies also suggest that strength-training programs
for children with cerebral palsy may help to increase muscle
strength and improve daily activities and quality of life
(Damiano et al. 1995; Dodd et al. 2002; McBurney et al.
2003; Morton et al. 2005). Furthermore, Suman et al.
(2001) conducted an intervention study in a group of children who had a total of greater than 40% of their body surface area burned. Patients were required to complete a 12week exercise program at home or in the hospital’s rehabilitation center. They were divided into two groups: the RT
group, which participated in an individualized training program supervised by personal trainers, and a control group
who were asked to complete a home-based rehabilitation
program without exercise. The results of this study showed
significant increases in muscle strength, total work resistance, and lean body mass in the RT group compared with
the control home group.
Based on anecdotal evidence, it was believed that RT lead
to injury of epiphyseal plates, cartilage, ligaments, or
muscles. However, prospective studies in children do not
support this belief. Faigenbaum et al. (2003) examined the
safety and efficacy of maximal strength testing in healthy
children between the ages of 6 and 12 years in a controlled
environment. During the intervention, the researchers asked
the children about muscle pain, soreness, and difficulty of
movements at the end of each testing session and over a period of time. The study concluded that during supervised
strength testing no injuries had occurred and no complaints
were reported in either the boys or the girls. In a previous
study, Weltman et al. (1986) examined the effectiveness
and safety of a 14-week hydraulic resistance-training program in 26 pre-pubertal males using musculoskeletal scintigraphy to assess tissue damage. They found no evidence of
damage to epiphyses, bone, or muscle as a result of strength
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Behm et al.
training and concluded that in the short term, supervised
concentric strength training using hydraulic resistance equipment is safe and effective in pre-pubertal boys (Weltman et
al. 1986). The safety of a resistance-training program in prepubescent to early post-pubescent males and females was
also examined by Lillegard et al. (1997). Only one injury
had been recorded during the 12-week training session. The
injury, a minor strain of the shoulder muscle, was considered incidental due to the low exercise to injury ratio and
the severity of this one injury (Lillegard et al. 1997). Injuries of the epiphyseal plates have been suggested to be less
likely to occur during childhood than during adolescence,
because the growth plates of children may actually be stronger and more resistant to various forces than those of adolescents (Micheli 1988). Further, although elite RT sports
such as gymnastics have also been associated in the past
with delayed growth and skeletal maturity, recent research
has shown that the shorter stature found in young gymnasts
when compared with age-matched controls is a result of selection rather than an effect of training on physical growth,
because of an advantage of shorter athletes in the sport
(Daly et al. 2000; Erlandson et al. 2008; Gurd and Klentrou
2003).
Recent studies appear to have come to a consensus regarding the beneficial effects of RT in young populations.
Not only has it been found to be beneficial to healthy growing muscles and bones, but it has also been found to help
children suffering from various diseases or health conditions. On the other hand, there are many precautions that
must be considered when practicing RT with children, the
most important being proper technique and appropriate volume. As pointed out by Selvadurai et al. (2002), one must
remember that even though RT aims at improving muscle
strength, other forms of physical activity such as cardiorespiratory activities should be practiced on a regular basis
to maintain a balanced and healthy lifestyle, optimize recovery time, and improve cardiovascular growth and function.
Thus, there are numerous beneficial effects of RT in general, and in children in particular. Most notably these include an increase in muscle strength (Blimkie 1992, 1993;
Falk and Tenenbaum 1996; Payne et al. 1997; Sale 1989).
Other beneficial effects include a potential increase in bone
strength, a desirable change in body composition, and an improvement in motor skills and sports performance. The next
section focuses on the physiological mechanisms explaining
the increase in muscle strength, highlighting the available
evidence in children and the known differences between
children and adults.
Physiological mechanisms
There are two generally acceptable types of adaptations
that may occur in response to RT and may explain the observed strength gains: morphological and neurological. The
relative contribution of these adaptations may be different
in children, adolescents, and adults.
Morphological adaptations
Morphological changes following RT include an increase
in muscle size, primarily due to an increase in fibre size, potential hyperplasia, and changes in fibre-type composition
551
and connective tissue, as well as structural changes in the
muscle. Commonly, morphological changes imply that
muscle mass has increased or hypertrophy has occurred.
This has been a common observation in adults, but not so
children or adolescents. Although RT has been shown effective in increasing muscle strength in children and adolescents the reported increases in muscle size have been
relatively small among studies. RT programs do not seem
to influence growth in height and weights of pre- and early
adolescent youth, whereas changes in body composition,
considering both fat and muscle mass, are minimal (Malina
2006; Falk and Eliakim 2003; Sadres et al. 2001). Studies
examining whole-muscle hypertrophy in children and adolescents have usually used anthropometric techniques and
have provided very limited evidence of hypertrophy in adolescents (Lillegard et al. 1997), and no evidence of muscle
hypertrophy in children (Blimkie 1989; Ozmun et al. 1994;
Ramsay et al. 1990; Sailors and Berg 1987; McGovern
1984; Siegel et al. 1989), as a result of RT. However, two
studies in which more sensitive methods of measurements
were used (magnetic resonance imaging and ultrasound)
have suggested that muscle hypertrophy may indeed occur
among children following RT. Mersch and Stoboy (1989)
used magnetic resonance imaging and were the first to demonstrate an increase in quadriceps cross-sectional area, together with increases in knee-extension isometric strength,
in pre-adolescent boys. However, only two sets of twins participated in this study. Later, Fukunaga et al. (1992) used
ultrasound to demonstrate increases in lean (muscle and
bone) cross-sectional area among 1st–3rd grade Japanese
boys and girls who engaged in RT (elbow flexion) over
12 weeks, whereas little change was observed in those children who did not train. Elbow flexors’ cross-sectional area
significantly increased, but interestingly, the extensor’s
cross-sectional area increased to a similar extent. Given
the small sample size in the study by Mersch and Stoboy
(1989) and the somewhat inconsistent results of the study
by Fukunaga et al. (1992), it may be premature to conclude that whole-muscle hypertrophy does indeed occur in
children as a response to RT. However, these two studies
do present the prospect that muscle hypertrophy is possible
among children, although these small potential changes
may be difficult to measure.
In the above studies, the anatomical cross-sectional area
was measured. In both of the studies that suggested hypertrophy in children (Mersch and Stoboy 1989; Fukunaga et
al. 1992), as is the case in most studies examining hypertrophy in adults, the increases in muscle cross-sectional area
were much smaller than the increases in muscle strength. In
other words, there was an increase in strength per whole
muscle area, sometimes referred to as muscle-specific tension. Theoretically, cross-sectional area should be measured
perpendicular to the line of pull of the fibres, called the
physiological cross-sectional area. However, this measurement is problematic and has not been attempted in children
or adolescents following RT.
The increase in the cross-sectional area of muscle as a result of RT in adults is primarily due to the hypertrophy of
individual muscle fibres (McDonagh and Davies 1984; Jones
et al. 1989). Changes in fibre cross-sectional area in humans
can only be examined using muscle biopsies. Given ethical
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552
considerations, it is understandable why such training-induced
data do not exist in healthy children and adolescents. Nevertheless, if muscle hypertrophy does occur in children, it is
likely due primarily to fibre hypertrophy. The latter is a result
of myofibrillar growth (an increase in contractile proteins)
and proliferation (an increase in the number of myofibrils), as
well as satellite cell activation in the early stages of RT
(Folland and Williams 2007). These mechanisms have not
been investigated in children or adolescents.
The occurrence of hyperplasia as a result of RT remains
controversial, but it has been suggested to take place in
adults following such training (Appell et al. 1988; Kadi and
Thornell 2000). However, this potential hyperplasia is argued to occur at a very slow rate and its contribution to
strength gains is argued to be minimal (Appell 1990). In
view of the need for muscle biopsy samples to investigate
this issue, hyperplasia has not been examined in children.
Other potential morphological effects of RT, which may
explain increases in muscle strength, include changes in myosin heavy-chain and fibre-type composition, increased tendinous stiffness, and an increase in the angle of muscle
pennation. Several studies have reported an increase in the
number of type IIa fibres and a concomitant decrease in
type IIx fibres in adults (Campos et al. 2002; Hakkinen et
al. 1998; Hather et al. 1991; Staron et al. 1990), suggesting
subtle fibre-type changes. These have not been examined in
children or adolescents. Tendinous stiffness has been demonstrated to increase following RT in adults (Kubo et al.
2001, 2002; Reeves et al. 2003), reducing the electromechanical delay in the muscle and increasing the rate of
force development. Although musculo–tendinous stiffness
has been reported as lower in children cthan in adults in
some (Lambertz et al. 2003) but not all studies (Cornu and
Goubel 2001), the effect of RT on tendinous stiffness in
children and adolescents has not been investigated. Finally,
recent studies in adults have provided strong evidence for
an increase in the angle of pennation following RT (Aagaard
et al. 2001; Kanehisa et al. 2002; Kawakami et al. 1995;
Reeves et al. 2004), allowing for more myofibrillar packing
and effectively increasing the physiological cross-sectional
area. An increase in the angle of pennation by itself is not
necessarily advantageous. However, an increase in myofibrillar packing would, in effect, increase muscle strength,
since most muscles in humans have an angle of pennation
substantially lower than the optimal 458. Although tendinous
stiffness and angle of pennation can be examined using noninvasive techniques, the effects of RT on these characteristics have not been examined in children.
Regardless of their potential existence in children, adolescents or adults, the morphological adaptations described
above explain only a small portion of the increases observed
in muscle strength among children and adolescents. More
studies using sensitive techniques are needed to clarify the
contribution of the various morphological adaptations to the
strength gains observed in children following RT.
Neurological adaptations
In view of the limited evidence of muscle hypertrophy
and its small potential contribution, strength gains among
children have been attributed mainly to neurological adaptations. These adaptations are difficult to define but can be
Appl. Physiol. Nutr. Metab. Vol. 33, 2008
viewed as modifications in coordination and learning that facilitate better recruitment and activation of muscles involved
in specific strength tasks (Folland and Williams 2007; Sale
et al. 1983). Measurement of such adaptation is elusive, and
therefore neurological adaptations are mainly based on indirect evidence.
In adults, indirect evidence of neural adaptations includes
the disproportionately greater increase in muscle strength
compared with the observed increases in muscle size. The
case is similar in adolescents, where some hypertrophy has
been demonstrated, but not sufficient to explain the increase
in muscle strength. In children, since there is minimal evidence of an increase in muscle size, the neurological adaptations are inferred from strength gains that are not
accompanied by muscle hypertrophy. In most cases, whether
children, adolescents, or adults, there is an increase in the
specific tension (torque/size) of the muscle. However, as
pointed out recently by Folland and Williams (2007), this
increase in specific tension can be explained not only by
neurological adaptations, but also by some morphological
adaptations, such as increases in tendinous stiffness or in
the angle of pennation.
No studies have specifically examined neurological adaptations in adolescents. There are only two studies that attempted to directly demonstrate neurological changes in
children following RT. Using the interpolated twitch technique, Ramsay et al. (1990) demonstrated an increase of 9%
and 12% in motor unit activation of the elbow flexors and
knee extensors, respectively, following 10 weeks of RT and
an additional 3% and 2%, respectively, following another
10 weeks of training. Nevertheless, the training-induced increases in strength were much greater than the concurrent
increases in neuromotor activation. Likewise, Ozmun et al.
(1994) used integrated electromyography amplitude (IEMG)
to demonstrate an increase in neuromuscular activation in
agonist muscles following 8 weeks of RT in pre-pubertal
boys and girls. As with the interpolated twitch technique,
the increase in IEMG was smaller than the increases in
strength (16.8% vs. 27.8%, respectively).
An increase in agonist’s activation is likely to result in
enhanced force production. However, the latter would also
be a result of a decrease in antagonist activation, or improved inter-muscular coordination. Several studies have
demonstrated lower antagonist co-activation in strength and
(or) power in adult athletes compared with non-athletes
(Baratta et al. 1988; Osternig et al. 1986). Similarly, some
studies have indicated lower antagonist co-activation in
adults compared with children (Frost et al. 1997; Lambertz
et al. 2003). Isometric training has been shown to decrease
antagonistic co-activation during knee extension in adults
(Carolan and Cafarelli 1992), but there are no comparative
studies in children or adolescents. This type of adaptation
likely has a greater influence on strength improvements in
complex multi-joint movements, rather than in simple
single-joint tasks.
Neurological adaptations are believed to occur predominantly in the early phases of training (Moritani 1992; Sale
1989). This is supported by the findings of Ramsay et al.
(1990) of greater increase in motor unit activation in children in the first 10 weeks of training, compared with the
second 10 weeks, as cited previously. In fact, the earliest
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phase of training likely involves the learning or optimization
of inter-muscular coordination (agonists, synergists,
stabilizers). Folland and Williams (2007) propose that the
magnitude of this learning depends on prior physical activity
level and experience in the specific task. This would suggest
that children, being younger and generally less experienced
or skillful in most tasks than adults, would exhibit greater
neurological adaptations in response to RT. Indeed, based
on the lack of observed morphological changes in children,
this notion has been indicated in the past (Blimkie 1989;
Sale and Spriet 1996). The specificity of training has not
been investigated in children. In adults, a low-repetition –
high-load RT program is recommended to increase maximal
strength. However, in 5- to 12-year-old children,
Faigenbaum et al. (1999) demonstrated that high-repetition –
low-load and low-repetition – high-load RT programs resulted in a similar enhancement of maximal strength. It is
therefore unclear whether the neurological adaptations to
RT in children are specific to the training parameters, as
would be expected in adults.
Thus, training-induced strength gains in children and adolescents may possibly be explained in part by muscle hypertrophy, but especially in children, are largely explained by
neurological adaptations such as increased motor unit activation or other changes such as improved inter-muscle coordination or neuromuscular learning (Kraemer et al. 1989;
Ozmun et al. 1994; Ramsay et al. 1990). The latter probably
has a higher relative contribution in more complex, multijoint actions (e.g., squat) than in single isometric contractions (e.g., of the knee extensors). The muscle learns to be
more efficient due to this stimulus and it is not until puberty
that the learned adaptation becomes permanent in the hypertrophic muscle (Malina 2006).
In view of the scarcity of findings, more research is required to elucidate the effect of different modes of training,
different training parameters (volume, intensity, frequency,
duration), and the status of maturity on the neurological
adaptations to RT in children and adolescents, along with
the morphological changes that possibly accompany these
adaptations.
Training guidelines and considerations
Youth RT programs need to be carefully prescribed and
progressed due to inter-individual differences in physical
maturation, training experience, and stress tolerance.
Although there is no minimal age requirement for participation in a youth RT program, all participants should have a
desire to resistance train and should be able to follow coaching instructions and comply with safety rules. In general, if
a child is ready for sports participation (generally age 7 or
8 years), then he or she may be ready for some type of RT.
A pre-participation medical exam is not required for apparently healthy children, but is recommended for youth with
known or suspected health problems (e.g., diabetes, obesity,
orthopedic ailments).
With age-appropriate instruction and competent supervision, regular participation in a youth RT program can offer
observable health and fitness value to boys and girls and
may foster favorable attitudes towards lifelong physical activity. However, over-prescription of RT and excessive pres-
553
sure from coaches and parents to perform at a level beyond
one’s capabilities may result in overtraining, injury, or burnout (American Academy of Pediatrics 2000; International
Federation of Sports Medicine 1998). The prescription of
RT programs should take into consideration the maturational
status of the youth, training mode, and extent and intensity
of other activities. It is not uncommon for some youth to be
involved in a number of sports or activities, which may limit
the possible positive training adaptations that could be accrued from additional RT. The training and participation in
multiple sports and activities highlight the need for periodized youth RT programs, which vary in volume and intensity throughout the season and year. For that reason, adult
exercise guidelines and training philosophies should not be
imposed on youth, since they are physically and psychologically less mature than adults.
Participation in a youth RT program should provide all
participants with an opportunity to learn about their bodies,
experience the benefits of resistance exercise, embrace selfimprovement, and feel good about their performances. In addition, youth RT programs can include basic education on
proper nutrition, adequate sleep, fitness conditioning, and, if
age-appropriate, performance-enhancing drug abuse. As
such, the cognitive and physical maturity of each participant
along with individual needs, goals, and abilities must be
carefully considered. Since enjoyment has been shown to
mediate the effects of youth physical activity programs
(Dishman et al. 2005), the importance of creating an enjoyable exercise experience for all participants should not be
overlooked.
A key factor in the design of any youth RT program is
appropriate program design, which includes instruction on
proper lifting techniques, correct prescription of the program
variables, and specific methods of progression. Since the act
of RT itself does not ensure that optimal gains in health and
fitness will be realized, youth RT programs need to be individually prescribed and sensibly progressed over time. Several specific areas of concern are important to consider
when designing youth RT programs; the quality of instruction, type of warm-up, choice of exercise, training intensity
and volume, and method of testing. The following is a summary of general RT guidelines for children and adolescents:
. qualified professionals (e.g., Certified Exercise Physiologists or Certified Strength and Conditioning Specialists)
should provide instruction and attentive supervision;
. consider each participant’s cognitive development, physical maturity, and training experience;
. ensure the exercise environment is safe and free of hazards;
. begin each session with a 5 to 10 min dynamic warm-up
period.
. start resistance training 2 or 3 nonconsecutive days/week;
. begin with 8–12 exercises that strengthen the upper body,
lower body, and midsection;
. initially perform 1 or 2 sets of 8–15 repetitions with a
light to moderate load (about 60% 1RM) to learn proper
form and technique;
. focus on learning the correct exercise technique and safe
training procedures instead of the amount of resistance or
weight lifted;
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.
.
.
.
include specific exercises that require balance and coordination;
gradually progress to more advanced movements that enhance power production;
cool down with less-intense activities and static stretching;
systematically vary the training program over time to optimize gains and reduce boredom.
Quality of instruction
Health and fitness professionals who have a thorough
understanding of youth RT guidelines and safety procedures
should provide instruction and supervision for all participants. In addition, professionals should genuinely appreciate
the developmental uniqueness of youth and should be able
to present information to children and adolescents in a way
that is appropriate for their level of understanding. Qualified
instruction not only enhances participant safety, but direct
supervision of youth RT programs can result in greater program adherence and increased strength gains as compared
with unsupervised training (Coutts et al. 2004). Although
adults with less experience may assist professionals in the
organization and administration of youth RT programs, it is
unlikely that they will be able to provide the level of instruction and supervision that is needed for safe and effective training. Professional certification in the area of
strength and conditioning (e.g., Certified Exercise Physiologists or Certified Strength and Conditioning Specialists) is
highly desirable.
Professionals need to be aware of the inherent risks associated with RT and should attempt to decrease this risk by
matching the RT program to the needs and abilities of each
participant. This is particularly important for untrained children who often overestimate their physical abilities (Plumert
and Schwebel 1997). An advanced RT program for an adolescent athlete would be inappropriate for an untrained child
who should be provided with an opportunity to learn basic
training procedures and experience the mere enjoyment of
resistance exercise. It is always better to underestimate the
physical abilities of a child rather than overestimate them
and risk negative consequences such as an injury.
Type of warm-up
All participants should warm up prior to RT. While a general warm up of low-intensity aerobic exercise and static
stretching is a common practice prior to participation in
recreational activities and athletic events (Martens 2004;
Shehab et al. 2006; Virgilio 1997), long-held beliefs regarding the routine practice of pre-event static stretching have
been questioned (Rubini et al. 2007; Shrier 2004; Thacker
et al. 2004). Recently, the effects of warm-up procedures
that involve the performance of dynamic movements (e.g.,
lunges, skips, twists, and throws) designed to elevate core
body temperature, enhance motor unit excitability, improve
kinesthetic awareness, and maximize active ranges of motion have received increased attention (Faigenbaum and
McFarland 2007; Verstegen and Williams 2004). Of note, a
dynamic warm up does not involve bouncing-type ballistic
movements, but rather a controlled elongation of specific
muscle groups.
Dynamic warm-up protocols that require balance, coordi-
Appl. Physiol. Nutr. Metab. Vol. 33, 2008
nation, power, and speed have been shown to enhance performance in children and adolescents (Faigenbaum et al.
2005a; 2006a; 2006b; Siatras et al. 2003), whereas pre-event
static stretching has been shown to reduce lower-extremity
power and isokinetic peak torque in youth (McNeal and
Sands 2003; Zakas et al. 2006). Furthermore, dynamic
warm-up procedures require participants to become immediately engaged in class activities and ready to listen to instruction (Graham 2001). Since chronic static stretching is
still recognized as a health-related component of physical
fitness in physical education programs (National Association
of Sport and Physical Education 2005), a reasonable recommendation is to perform dynamic activities during the warmup period and static stretching exercises that are relaxing
and less intense during the cool-down session. These recommendations are consistent with others who suggest that static
stretching should be performed after exercise (Fields et al.
2007; Shrier 2004).
Choice of exercise
A limitless number of exercises can be used to enhance
muscular fitness provided that the exercises are appropriate
for a child’s body size, fitness level, and exercise technique
experience. Weight machines (both child- and adult-sized),
free weights (barbells and dumbbells), elastic bands, medicine balls, and body mass exercises have been shown to be
safe and effective for children and adolescents (Annesi et al.
2005; Faigenbaum and Mediate 2006; Faigenbaum et al.
2005b; Falk and Mor 1996; Ramsay et al. 1990; Sadres et
al. 2001; Siegel et al. 1989). When deciding on equipment,
realize that adolescents may be able to use adult-size weight
machines, but that small children will not be able to position
themselves properly on these large machines. Since children’s smaller body size usually precludes the use of adultsized equipment, child-size machines or other modes of
training (e.g., dumbbells or medicine balls) are most appropriate for small children. Single-joint exercises (e.g., biceps
curl and leg extension), which target specific muscle groups,
and multi-joint exercises (e.g., bench press and back squat),
which involve the coordinated action of many muscle
groups, can be incorporated into a youth RT program. Regardless of the choice of exercise, the concentric and eccentric phases of each lift should be performed in a controlled
manner with proper technique.
For youth beginning RT, it is important to choose exercises that match abilities. As such, it is reasonable to start
RT with simple exercises and gradually progress to more
complex exercises as competence and confidence improve.
Advanced multi-joint exercises including Olympic-style lifts
(e.g., snatch and clean and jerk) and modified cleans, pulls,
and presses may be incorporated into a youth RT program
(Faigenbaum et al. 2007a; Sadres et al. 2001). With qualified coaching and safety measures in place (e.g., safe lifting
environment, appropriate loads), data indicate that risk of injury during the performance of Olympic-style lifts during
training and weightlifting competition is relatively low
(Byrd et al. 2003; Hamill 1994; Pierce et al. 1999). Nevertheless, Olympic-style lifts involve a more complex neural
activation pattern and therefore participants need to learn
how to perform these lifts early in the workout with a relatively light load (e.g., wooden dowel or unloaded barbell) to
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Behm et al.
develop coordination and skill technique without undue fatigue. As neural or learning adaptations are generally accepted as the major contributor to strength gains during
preadolescence, the progression to more complex coordinated movements including Olympic-style lifts may be permitted during this developmental period to potentially
enhance neuromuscular organization.
Plyometric training or stretch–shortening cycle exercise
can be safe and effective for enhancing muscle power in children and adolescents provided that appropriate training and
guidelines are followed (Brown et al. 1986; Kotzamanidis
2006; Lephart et al. 2005; Marginson et al. 2005; Matavulj
et al. 2001; Diallo et al. 2001). Past recommendations for
adult plyometric training (i.e., the athlete should be able to
squat at least 1.5 times his or her body weight before performing lower-body plyometrics (Potach and Chu 2000) may
have inhibited the implementation of plyometric training for
youth. ALthough these adult recommendations may be appropriate for high-intensity or high-amplitude plyometrics,
children and adolescents regularly perform plyometrics when
they skip, hop, run, bound, and jump.
Typically, plyometric training involves body mass jumping exercises and medicine ball throws that are performed
quickly and explosively. With plyometric training, the
neuromuscular system is conditioned to react more quickly
to the stretch–shortening cycle. Thus, this type of training
may enhance a young athlete’s ability to increase speed of
movement and improve power production (Chu et al. 2006).
Youth should begin plyometric training with less-intense
drills (e.g., double-leg jumps) and gradually progress to
more advanced drills (e.g., single-leg hops) as competence
and confidence to perform this type of training improve.
Studies indicate that relatively few repetitions (i.e., £10) of
each plyometric drill are needed to bring about significant
training-induced gains in performance (Lephart et al. 2005;
Myer et al. 2005; Matavulj et al. 2001). Plyometric training
should take place on yielding surfaces (e.g., gymnasium
floor or playing field) and the focus of early training should
be on proper athletic positioning and landing. Since plyometric training is not intended to be a stand-alone exercise
program, the best approach is to incorporate this type of
training into a well-rounded program that also includes other
types of strength and conditioning (Faigenbaum et al. 2007b;
Ingle et al. 2006; Myer et al. 2005).
Exercises that require balance should also be incorporated
into youth RT programs, since balance is essential for optimal performance and the prevention of athletic injuries
(Verhagen et al. 2005). In adults, balance is related to the
ability to exert force and power and therefore the ability to
maintain and (or) control a body position can enhance the
neuromuscular adaptations to RT (Anderson and Behm
2004). Typically, a stiffening strategy that decreases the
magnitude and rate of voluntary movements is adopted
when adult participants are presented with a threat of instability (Adkin et al. 2002; Carpenter et al. 2001). Thus, an
RT program that includes exercises, which could improve
stability or balance, could subsequently enhance force output, power, and coordination. In support of these observations, significant correlations between skating performance
and the static wobble board balance test have been reported
in youth under 19 years of age (Behm et al. 2005b).
555
Given that balance and coordination are not fully developed in children (Payne and Isaacs 2005), balance training
may be particularly beneficial for reducing the risk of injury
while performing RT, particularly to the lower back. A number of studies in adults have demonstrated increased muscle
activation of trunk muscles when performing activities on an
unstable versus a stable surface (Anderson and Behm 2005;
Behm et al. 2005a). The advantage of training on an unstable surface is that high activation can be achieved without
the imposition of high resistive loads (Anderson and Behm
2004; Behm et al. 2005a). When incorporating balance training into a child’s or adolescent’s RT program, exercises
should progress from simple static balance activities on stable surfaces to more complex static instability training using
devices such as wobble boards, BOSU (‘‘both sides up’’)
balls, and stability balls (Behm and Anderson 2006). Over
time, the program can be made more challenging by changing the base of support, the moment or lever arm of the body
segment, the movement pattern, or the speed of motion.
Training intensity and volume
RT intensity refers to the amount of weight lifted during
the performance of an exercise, whereas training volume is
typically estimated from the number of exercises performed
per session, the repetitions performed per set, and the number of sets performed per exercise. Training intensity and
training volume have a direct impact on training adaptations
and are dependent upon other factors including exercise order, repetition speed, and rest interval length (Kraemer and
Ratamess 2004).
Different combinations of sets and repetitions from singleset protocols with a moderate load (Westcott 1992) to progressive training regimens consisting of 3–5 sets with loads
ranging from 70% to 85% of 1RM have proven safe and effective for youth (Ramsay et al. 1990). Although there is not
one combination of sets and repetitions that will be optimal
for all participants, a reasonable approach is to begin RT
with 1 or 2 sets of 8–15 repetitions with a light to moderate
load (30%–60% 1 RM) on 8 to 12 exercises. A training frequency of at least 2 nonconsecutive days/week is recommended, as RT only once per week may result in
suboptimal adaptations (Faigenbaum et al. 2002). This type
of program will provide an opportunity for beginners to
learn proper lifting techniques while maximizing gains in
muscular strength (Faigenbaum 2000; Kraemer and Fleck
2005).
Youth with RT experience can gradually progress to more
intense or voluminous workouts to target specific training
objectives (i.e., strength, power, hypertrophy, and (or) muscular endurance). For example, the performance of 3 sets
with heavier loads (e.g., 6–10RM) performed to volitional
fatigue can be used to increase maximal strength on large
muscle group exercises (e.g., leg or bench press). Depending
on program goals and individual abilities, progression can
also be achieved by enhancing movement speed during the
performance of selected exercises (i.e., plyometric drills and
Olympic-style lifts). It is important to note that not all exercises need to be performed for the same number of sets and
repetitions and that in some cases less-intense training can
provide needed variation during long-term athletic-training
programs.
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Although additional long-term training studies are needed
to explore the effects of different RT programs on youth, the
best approach is to vary the RT program over time to keep
the training stimulus challenging and effective. This does
not mean that every training session needs to be more intense or voluminous than the previous session, but over
time the RT program needs to be systematically varied to
stimulate further adaptations and maximize gains (Kraemer
et al. 2002). In the long term, program variation with adequate recovery between training sessions will allow children and adolescents to make even greater gains because
their body will be able to adapt to even greater demands
(Bompa 2000; Kraemer and Fleck 2005).
Method of testing
Strength testing provides an opportunity for professionals
to assess initial strength levels, identify muscle imbalances,
develop individualized programs, and monitor progress. In
addition, if presented and administered properly, strength
testing can provide an incentive for young participants to
resistance train regularly to improve their strength performance. Although there are a variety of methods for evaluating
muscular strength in children and adolescents (Gaul 1996),
researchers typically used maximal load lifting (e.g., 1RM),
relatively high RM lifting (e.g., 10RM), and maximal isokinetic tests to assess muscle strength in youth (Benson et
al. 2007; Faigenbaum et al. 2003; Lillegard et al. 1997;
Pfeiffer and Francis 1986; Ramsay et al. 1990). No injuries
have been reported in any prospective youth RT study that
involved strength testing procedures. It should be underscored that strength testing in the aforementioned reports involved adequate warm-up, gradual progression of testing
loads, and close and competent supervision and instruction.
Although strength testing is not a prerequisite for participation in a youth RT program, professionals who have experience testing youth can administer strength tests to evaluate
training-induced gains in muscular strength and muscular
endurance. While field-based measures (e.g., push-up or
modified pull-up) are appropriate for testing a large group
of children (e.g., a physical education class), the use of RM
strength testing procedures may provide more useful information for professionals who need to assess strength performance in trained youth (e.g., a youth sports program). Of
note, RM testing procedures are labor intensive, time consuming, and require close, qualified supervision. Unsupervised and poorly performed strength tests should not be
carried out under any circumstances because of the potential
for injury.
Risks and concerns
A traditional concern associated with youth RT involves
the potential for injury to the epiphyseal plate or growth
cartilage. Although this type of injury is possible if proper
training guidelines are not followed (Gumbs et al. 1982;
Jenkins and Mintowt-Czyz 1986), an epiphyseal plate fracture has not been reported in any prospective youth RT
study that was competently supervised and appropriately
progressed. If children and adolescents are taught how to resistance train properly, it seems that the risk of injury to the
growth cartilage is minimal. Moreover, data suggest that
regular participation in a well-designed RT program does
Appl. Physiol. Nutr. Metab. Vol. 33, 2008
not negatively impact growth or maturation of youth (Falk
and Eliakim 2003; Malina 2006). Traditional fears associated with youth RT have been replaced with more recent
findings that indicate that regular participation in weightbearing physical activities is essential for normal bone
growth and development (Bass 2000; Vicente-Rodriguez
2006).
It seems the greatest concern for children and adolescents
who resistance train is the risk of an overuse soft-tissue injury, particularly to the lower back (Brady et al. 1982;
Brown and Kimball 1983; Risser et al. 1990). These observations are consistent with other data, which suggest lower
back pain is the number one musculoskeletal problem in
North American adults (Coyte and Ashe 1998). Since weak
musculature, improper lifting techniques, or improperly designed RT programs may explain, at least in part, these observations, professionals need to be aware of the inherent
risks associated with RT and should attempt to decrease this
risk with proper instruction and program design. As such,
professionals should include progressive strengthening exercises for the hips, abdomen, and lower back in youth RT
programs as part of a preventative health measure.
All types of physical activity carry some degree of risk of
musculoskeletal injury, but the risk of injury resulting from
RT can be minimized with appropriate overload, gradual
progression, careful selection of exercises, and adequate recovery between training sessions. Of note, youth should not
resistance train on their own without guidance from qualified professionals and, when appropriate, a spotter should
be nearby in case of a failed repetition. Each participant
must be treated as an individual due to the variability in
children and adolescents of the same age to tolerate stress.
Prescribing an RT program that exceeds a child’s ability
may undermine enjoyment of the training experience and
may increase the risk of an acute or overuse injury. Qualified supervision, age-appropriate program design, safe exercise equipment, and a clean training environment are
paramount.
Conclusions
In summary, a properly supervised and instructed RT program using appropriately sized equipment, involving exercises within the child’s or adolescent’s capability, and
employing gradual progression can be implemented for
youth. It has been well documented that optimal growth and
development of the musculoskeletal system is achieved
when progressive overload stresses are placed on the system.
RT is one activity that can provide these results, whereas
other sport and play activities that involve dynamic movement of body mass over extended periods can also provide
positive adaptations. RT exercises can range in complexity
from simple body mass, dumbbell, or machine-type resistance exercises to more advanced techniques such as plyometrics, instability RT devices, and Olympic-style lifting.
Training-related physiological adaptations include neurological adaptations with an emphasis on learning and
co-ordination, with limited evidence of muscle hypertrophy. However, more research is necessary regarding
the physiological mechanisms of strength gains in children and adolescents as a result of RT. These mecha#
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Behm et al.
nisms include, but are not limited to, muscle hypertrophy,
hyperplasia, fibre-type transformation, changes in tendinous stiffness, angle of pennation, motor unit recruitment,
muscle activation, and antagonist co-contractions. Implementing an RT program for children and adolescents may
not only improve muscular strength, endurance, power,
and balance; there is evidence for improvements in body
composition and motor skills, as well as functional performance improvements for individuals coping with cystic
fibrosis, cerebral palsy, and burns.
Acknowledgement
Writing of this Position Stand was made possible by an
unrestricted education grant to the Canadian Society for Exercise Physiology by Merck Frosst.
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